The Supreme Court today upheld the 2010 health care law, validating President Barack Obama’s top domestic policy achievement while dealing a stinging rebuke to mostly Republican critics who charged the statute was unconstitutional.

The opinion gives legal certainty to one of the most sweeping pieces of domestic policy legislation since President Lyndon B. Johnson’s Great Society programs in the 1960s and allows insurers, hospitals, drugmakers and other health care providers to continue planning for sweeping changes, many of which are set to take place in 2014.

Twenty-six states and the National Federation of Independent Business challenged a provision in the law requiring most Americans to buy health insurance or pay fines, saying it exceeded legislators’ power over interstate commerce. States also challenged an expansion of Medicaid, a health care program for the poor.

Chief Justice John Roberts joined four liberal justices on the high court in ruling the coverage mandate could survive as a tax. The court limited the Medicaid expansion but didn’t invalidate the provision.

Other elements of the law remain intact, including a requirement that health care plans offer coverage to individuals with pre-existing conditions and provisions that allow young adults to stay on their parents’ insurance until age 26 and help close a gap in seniors’ Medicare drug coverage.